The present application relates to microchip laser devices.
Microchip lasers are miniature diode-pumped solid-state devices formed by dielectrically coating thin platelets of gain media. Microchip lasers have continuous wave output characteristics that are comparable to those of the best other types of solid state devices. A microchip laser 100, in its simplest implementation as shown in FIG. 1A, consists of a small piece of solid-state gain material 103 having two polished, flat parallel surfaces at the ends. Cavity mirrors 102,104 are dielectrically deposited onto the polished surfaces. In operation, the microchip laser 100 is longitudinally pumped with a diode laser 101. An important aspect of microchip lasers is that their end surfaces are flat, which can be accomplished by polishing and coating a large wafer (e.g. 3 inch wide) before the wafer is cut into small pieces (e.g. 1 mm square pieces) each forming a microchip laser. The fabrication simplicity and small sizes allow microchip lasers to be mass produced at low cost.
Microchip laser typically have their fundamental frequencies in the IR range. However, many applications, such as displays, biomedical instrumentation, and higher density optical storage systems, require laser output in the visible range. Intra-cavity frequency-doubled microchip lasers can produce laser beams in the blue, green, and red wavelength ranges. A frequency-doubled microchip laser 150, shown in FIG. 1B, can include a gain material 103 formed by Nd:YVO4 and a frequency doubling material 105 formed by KTP which are bonded together to form the laser cavity. The gain material 103 and the frequency doubling material 105 both have polished and parallel surfaces. The outer surfaces are coated by highly reflective films to form cavity mirrors 102,104. The cavity mirrors 102,104 reflect light at the 1064 nm fundamental wavelength. When the frequency-doubled microchip laser 150 is pumped by a laser diode 101, a strong circulating field is built up at 1064 nm. The cavity mode diameter is small, which results in a high circulating intensity. As the fundamental field passes through the frequency doubling material 105, a significant amount is converted to the second harmonic at the visible wavelength of 532 nm.
FIG. 1C shows a low-noise microchip laser device 180 that includes a gain material 103 and a frequency doubling material 105 which are bonded together and a quarter-wave plate 106 for the fundamental wavelength is inserted in the cavity between the mirror 104 and the frequency doubling material 105. When the frequency-doubled microchip laser 180 is pumped by a laser diode 101, a strong circulating field is built up at the fundamental frequency. As the fundamental field passes through the frequency doubling material 105, a portion of it is converted to the second harmonic at the doubling frequency. The fast axis of quarter-wave plate 106 and the optical axis of the frequency doubling material 105 can be set up at 45° relative to each other. The instability caused by the coupling of two polarization modes was effectively suppressed, yielding green output with low noise.
FIGS. 2A-2C show examples of far-field patterns formed by green laser beams of a microchip laser device. In FIG. 2A, a laser beam in TEM00 mode produces approximately a circular pattern. FIG. 2B shows the far-field pattern formed by a laser beam including both TEM00 mode and TEM01 mode. FIG. 2C illustrates the far-field pattern formed by a laser beam comprising TEM00 mode, TEM01 mode and TEM02 mode.
A laser beam in diffraction-limited TEM00 transverse mode showing compact circular beam spot pattern (as shown in FIG. 2A) is desirable for most applications of microchip lasers. However, due to the inherent characteristics of microchip lasers and especially the intra-cavity frequency-doubled microchip lasers, high-order transverse modes commonly appear in most microchip laser devices.
Microchip lasers have flat-flat cavities and plane waves in their eigen modes. Optical pumping creates heat in the microchip lasers. If the refractive index of the gain media increases with temperature (e.g. as in the case of Nd:YVO4), a waveguide can be formed. In addition, thermal expansion of the gain medium can result in end-face curvature. The end-face curvature can also result in a stable cavity mode. For these reasons, thermal effects can be beneficial in defining a stable cavity. On the other hand, thermal effects also limit the performance of the microchip lasers. One important advantage of the microchip lasers is that they form self-aligning, stable cavities in contrast to other solid-state lasers that require careful alignment of cavity mirrors. This advantageous feature, however, is affected by the thermal effects at low and high average pump powers. At low pump power (and low output power), the resonance cavity is poorly defined because low heat creation produces small thermal lensing effect. The microchip lasers operate in multi-transverse modes. The thermal effects can cause the operations of microchip lasers to be highly sensitive to temperature; microchip lasers often can operate in TEM00 mode only in narrow temperature ranges.
Furthermore, intra-cavity frequency-doubled lasers can be more prone to multi-transverse modes than conventional output coupling. Some researchers indicated “small amount of unextracted pump energy outside the 1/e2 radius of the TEM00 beam is usually not enough to support higher order modes. However, the nonlinear gain saturation of the intra-cavity frequency-doubled laser combined with the poor spatial overlap of the transverse modes increases the gain and reduces the nonlinear losses for the higher order modes.” (Anthon et al., J. Quantum Electronics, 28, 1148, 1992)
In intra-cavity frequency-doubled microchip lasers, the conversion efficacy of second harmonic is very high; this efficacy for TEM00 mode fundamental wave is much higher than that of higher order mode fundamental waves. As a result, nonlinear loss for the TEM00 mode is much larger than nonlinear losses for higher order mode, which makes the TEM00 mode unstable and the higher order transverse modes easily excited in intra-cavity frequency-doubled microchip lasers. The above described drawbacks in the intra-cavity frequency-doubled microchip lasers result in low manufacturing yield, narrow operation temperature ranges, and narrow output power range for operating in TEM00 mode in this type of lasers.
There is therefore a need to increase manufacturing yield and to expand temperature and power ranges for the operation of TEM00 mode in microchip lasers.